The present invention relates to methods and devices for demineralizing fluids, and relates to filtration or treatment cartridges or modules, having a generally cylindrical aspect and constructed with plies of spirally wound selectively-permeable membranes. It particularly relates to electrodialysis and electrodeionization devices, wherein the membranes possess ionic selectivity, and the device includes electrodes for inducing transport of species across the membranes by ionic conduction.
In general, electrodialysis (ED) and so-called electrodeionization (EDI) devices operate by providing a structure that arranges flow channels such that a flow of a feed fluid that is to be treated is channeled between two ion exchange membranes of opposite exchange type, while an electrical potential is applied across the membranes transverse to the flow to maintain an ionic current that demineralizes the feed fluid, moving ionizable species from the feed fluid in one channel, through the membranes, and into adjacent channels, thereby producing a demineralized product flow from the feed. Spacers position successive membranes apart to define the fluid treatment channels or “dilute” flow spaces.
A subclass of electrodialysis (ED) devices, often referred to as electrodeionization (EDI) devices, further include a packing of ion exchange material, typically beads or felt, as a flow-permeable packing within the flow treatment channels and, in certain constructions, within the adjacent mineral-receiving channels. The presence of exchange material in the treatment channels or cells enhances the active fluid interaction area and the capture of ions from the feed, and provides a stationary transfer medium of good electrical and ionic conductivity for transporting the captured ions to and across the surrounding membranes. This construction offers a robust and efficient mechanism for effectively separating many dissolved materials from the flow along a relatively short flow path. The ion exchange material is continuously maintained in an at least partially regenerated (active) state by water splitting.
Over many years, developers of these units have explored the suitability and operating characteristics of ED and EDI devices with a range of flow channel geometries and flow velocities, various membranes defining cells of different fixed or even progressive thickness, and a variety of ion exchange fillings distributed in various localized patterns (such as stripes, bands, special monotype or mixed beds) and other variations. For certain applications, the use of beads with special sorption, catalytic or other properties has been described to stabilize operating characteristics or effect other aspects of treatment.
In these devices, the feed fluid flows one or more times through “dilute” spaces or cells, giving up its ions, to emerge as a substantially demineralized or treated product flow, while a separate fluid in adjacent “concentrate” or “brine” cells receives the minerals stripped from the feed by ionic conduction through the membranes, together with such non-ionic small molecules as may pass through the membranes. Various physical implementations of ED and EDI units are known. The majority of commercial devices, particularly EDI devices, have historically employed an architecture based on flat plate “stacks” —arrangements of many cells formed by stacking substantially oblong membranes, spacers, and screens—collectively forming many cells—between endplates, with electrodes and usually ports or manifolds positioned at the ends of the stack. Similar stacks of disk-shaped cells are historically known. In addition to these “stack” constructions, many publications also describe, and several companies have commercially marketed, cylindrically-shaped ED or EDI devices having cells formed between ion exchange membranes that are spirally-wound about a pipe or core. These devices have electrodes at radially inner and outer positions to apply a substantially radial electrical field between the core and the outer shell of the cylindrical device.
ED (unfilled) devices have found use in treating a number of food industry fluids. A rolled spiral construction similar to the spiral ED or EDI units has also long been used in fabricating cross-flow reverse osmosis (RO), microfiltration (MF) and other types of filtration/separation modules for use with feed streams of alimentary fluids or fermentation product streams, so the spiral architecture is well accepted in that industry for its flow dynamic characteristics, plumbing requirements, ability to handle elevated pressure and other desirable properties. These other spiral-wound filtration devices typically rely upon elevated pressure to drive the filtration process or product through a membrane, rather than upon an electric potential to transport ionizable components across a membrane. Such spiral filter constructions typically permit only small deflections, and are able to sustain high pressures without rupturing membranes. Applicant believes that a spiral EDI construction may potentially enjoy a pressure resistant construction that would desirably permit enhanced throughput, longer, more effective treatment path length or other improved property.
Among the published or commercially promoted spiral ED and EDI products, early examples of Ionics, Incorporated, as shown in U.S. Pat. No. 2,741,591, describe various directions for the respective dilute and concentrate flows, both in relation to the inner and outer electrode and with respect to each other. The Christ, A. G. company of Switzerland has more recently marketed spiral EDI devices, of which examples are shown in their U.S. Pat. No. 5,376,253, entitled Apparatus for the Continuous Electrochemical Desalination of Aqueous Solutions, naming inventors Rychem et al. The construction shown in that patent is a spiral wound EDI with inner and outer electrodes, having its fluid treatment dilute cells sealed to the wall of, and opening into, the inner electrode (which also serves as a central flow pipe), and having its concentrate cells open to the surrounding cylindrical wall that forms a counter-electrode.
Another commercial EDI unit of spiral architecture, originally developed in China, employs mesh-filled wound concentrate envelope and provides an axially oriented dilute flow between the windings. This device is marketed in the United States by Omexell, Inc. of Houston, Tex. The Omexell device is illustrated in U.S. Pat. 6,190,528, naming inventors Xiang Li and Gou-Lin Luo. In that construction, a central pipe is both an electrode and a water distributor, while wound metal strip or wire forms the outer electrode. Two membranes surrounding a mesh web form an envelope without any exchange bead filling, and the envelope is spirally-wound about the central pipe to form the concentrate flow space(s) of the device. The alternate regions between successive turns of the envelope are filled with ion exchange resin beads to constitute the dilute channels. The input feed flow and the treated product output proceed through the exchange bead-filled space along an axial direction, from one end of the cylinder to the other, while the concentrate flows from the product feed inlet (embodiment #2, shown in FIG. 4 of the aforesaid '528 patent) or from a slot along half the central electrode/pipe (embodiment #1, shown in FIGS. 1-3 of that patent), along a helical path through the wound concentrate envelope and into (or back into) the central electrode/pipe. Thus, the Omexell construction winds a membrane/spacer/membrane concentrate envelope, and fills the space between windings with resin to form the dilute passages. The resin filling is stated to be replaceable.
Some spiral EDI devices may employ a central pipe as an electrode that doubles as a fluid manifold. Early flat plate EDI stacks were arranged with their dilute and concentrate flows in parallel planes but at a right angle to each other, or at a meandering angle with respect to each other, while many modem flat plate rectangular or oblong EDI stacks are now configured so that dilute and concentrate flows are arranged in closely-spaced parallel sheets in either a co-current or counter-current arrangement. Spiral EDI devices tend to arrange a major portion of the two flow paths cross-current, with one flow being axial and the other locally across the axis along a globally helical path following the spiral contour of the membrane envelopes that define the dilute and/or brine cells. The spiral architecture permits one to define different relative path lengths and flow rates of the two fluids (for example, the axial path may be shorter than the spiral path), and may allow some flexibility or advantages in other respects, such as ease of re-filling or refurbishment, over clamped-plate stack designs.
The Omexell spiral EDI construction is advertised as being readily serviceable, and the '528 patent mentions replacing the dilute cell exchange beads every day by opening the ends of the cylinder, blowing out the exchange beads, and re-filling. That Company has filed a number of This accessibility of the beads in the construction of the '528 patent has been advertised to promote the product by contrasting it to the situation applying to conventional stacks of rectangular construction mentioned above, in which the separate replacement of the exchange beads is generally either quite cumbersome (for example, requiring disassembly and re-assembly of the stack, or requiring a complex emptying and filling regimen), or else is not feasible (because the dilute cells are each formed as discrete permanently sealed envelope-cells that cannot be opened). However, it is not entirely clear from the '528 patent or from the commercial product description why bead replacement is deemed necessary. It is possible that the patent, being a short technical description drafted by a third party at an early stage of development, contains an erroneous description. It is also possible that the common practice in China of relying upon ion exchange beds for primary water treatment influenced the inventors to emphasize, in the '528 patent, the replaceability of exchange beads, so that the new EDI technology would be seen not as an unproven and different technology, but as simply an augmented form of the accepted and proven treatment involving periodic renewal of an ion exchange bed. It is also possible, however, that the device described in the '528 patent was prone to scaling as a result of the minerals (such as calcium and silica) present in the local waters and the nature of fluid flows and electrical fields within the device, and that resin replacement was necessary in that particular context.
EDI units were first developed forty or fifty years ago. At a historically early period of this development, the bead filling was often more or less readily accessible, and one could replace or regenerate the beads separately at frequent intervals to achieve a desired degree of treatment. This allowed the treatment regimen to rely in part on the bead storage capacity (like that of a conventional ion exchange bed) to accommodate part of the removal burden or to effectively remove certain ones of the less mobile ions. Generally, however, modem stacks and EDI devices are designed to operate without disassembly or resin replacement for extended times—a period up to several years. During operation, a portion of the exchange bead filling is continuously electrically regenerated, and the devices are operated in a steady state. While certain feed water quality standards may be specified to assure long term stability, occasional total regeneration and/or cleaning or reversal cycles my be performed to address scale-like build-up or performance deterioration, and to prevent any fouling or scaling from irreversibly impairing operation.
Without dwelling further on generalities or specific constructions, it may be said that EDI constructions of both the stack and the spiral architectures rely on the capture of ions by exchange beads and the transport of captured ions through a chain of one or more beads either to, or closer to, the exchange membranes that actually transfer the ions out of and separate the ions from the feed flow/dilute path. The exchange beads are continuously regenerated (for example, by hydronium or hydroxide ions that are created by water splitting at places of high field intensity, such as heterogeneous bead/bead or bead/membrane junctions), and the devices are generally set up to operate in a steady state on a given feed for extended periods of time. However, the rate or flow distribution and other factors governing all these effects are such that conditions of high concentration of specific ions, extreme pH, or flow stagnation may all arise in use, and certain combinations of these conditions may pose control problems, impair the efficiency or degree of treatment, or risk introducing irreversible membrane damage and/or localized occurrences of resin or membrane scaling within the device. The dimensions and geometry of the flow cells, the nature of the exchange filling formulations, and details of the hydraulic plumbing may all be important in addressing such problems, and a certain amount of pretreatment of the feed fluid is also generally required to assure a suitable initial feed quality that will not give rise to problems over the long term. Extensive industrial operating experience further allows one to specify operating parameters and protocols to follow for each device with various feeds in order to safely avoid, address or minimize long term performance deterioration.
One aspect of EDI device construction deserves special mention, namely that the membranes as well as the exchange beads employed in these devices are swellable, and generally undergo changes in dimension between their dry and hydrated forms. Some heterogeneous exchange membranes may swell by twenty percent, and wetted beds of exchange beads also increase their volume and may exert high pressure if unduly confined. Such swelling may impair the flow impedance, or may affect the integrity of membranes or structural elements. This has lead various manufacturers of EDI stacks to propose assembly steps such as pre-soaking membranes for lengthy periods before assembly; using more rigid intermediate frame or spacer assemblies having multiple lands, bosses, beads and/or registration pins to secure the membranes, confine the exchange beads and maintain alignment and sealing; filling of beads by precisely-measured quantities in a dry or salted form to achieve precisely quantified swelling, or filling as pre-formed blocks or gels of exchange media; or dynamic filling of cells by a fluidized and possibly salted slurry, to assure a desired cell packing.
For spiral constructions, the dimensional instability of membrane and bead media, together with the local slippage introduced by winding at different radii, and the relatively large length of individual membranes, raise additional potential problems of membrane spacing or support, stress, shrinking or buckling, and cracking. A number of investigators have proposed the use of fixed and pre-formed spacing elements such as bumps, posts or ribs rather than beads, either as separate elements, or as features formed on the membrane surface, to avoid irregular spacing or undue mechanical stresses and to maintain a desired membrane-to-membrane spacing.
Within this general picture, various problems or perceived problems or design constraints may arise. For example, in the 1960's it had been shown that certain properties of EDI operation are optimized with uniform sized ion exchange beads, and with thin filled cells; in the commercial field, some industry advocates have long urged that a cell thickness defined by a low number of exchange beads (e.g., 4-10 beads) is optimal. Thick cells have also been advocated for specific purposes, such as high silica removal achieved by inducing an upward pH shift due delayed hydroxyl removal under polarized operation. It is apparent that a small cell thickness introduces hydraulic flow limitations that will vary greatly as a function of exchange bead size and feed fluid viscosity; theoretical or empirical modeling done with water would not necessarily apply to systems for treating common alimentary fluids. Moreover, with any feed, local current density may vary within the many cells of a conventional EDI stack or device, and is substantially affected by local variations in distribution of exchange beads, as well as by channeling or local variations in flow that may occur. These current variations and resulting potentials may profoundly alter the intended operating performance. In addition, in spiral devices, current density increases inversely with radial position, raising further control or operational difficulties. Moreover, fluids such as alimentary or fermentation fluids are notoriously prone to fouling—both functional fouling of exchange bead surfaces and functionality, and physical blockage of flow through the exchange beds. Fluidized exchange beds have been employed to address the latter problem, but this approach cannot be employed with the exchange bead filling of EDI devices, because it is inconsistent with the requirement of direct contact between exchange beads and the constricted space existing between the exchange membranes.
For such reasons, the fabrication and operation of EDI demineralization devices remain rather complex and costly, and each particular construction may have its own limitations or drawbacks.
There is thus a need for new constructions of such devices, for devices that offer improved cost or ease of manufacture, and for EDI devices that provide different or improved operating abilities.